The present invention relates to a recording/reproducing head positioning mechanism for a disc apparatus, such as a magnetic disc apparatus or an optical disc apparatus and, more particularly, to a small size, highly accurate magnetic head positioning mechanism, which is a two-stage actuator comprising a fine actuator including a head gimbal assembly with a magnetic head provided thereon at one end thereof, and a coarse actuator with the fine actuator supported thereon and secured thereto, and has satisfactory vibration characteristics free from main resonance up to a high frequency band and excellent response characteristics.
The recording density of the magnetic disc apparatus is increasing at a pase of 60% or above per year with increasing BPI (Bit Per Inch) and TPI (Track Per Inch).
For increasing the BPI, reduction of the head float-up, adoptation of high sensitivity magnetic heads such as MR (Magneto Resistive) heads and highly efficient signal processing techniques are required. To realizeTPI increase, it is an important additional technical subject to improve the accuracy of magnetic head positioning. For an example, while with a recording density of 1 Gb/in.sup.2, the track density is 8 kTPI or below corresponding to 3 to 4 .mu.m, for attaining the recording density of 10 Gb/in.sup.2 the track density amounts to 25 kTPI or above corresponding to a track pitch of 1 .mu.m or above. In this case, the accuracy of magnetic head positioning should be 0.1 .mu.m or above (which is approximately 10% of the track pitch).
A magnetic head positioning mechanism (i.e., positioner) used in the prior art magnetic disc apparatus is shown in FIGS. 36 to 39. FIGS. 36(a) and (b) are plan views showing the prior art magnetic head positioning mechanism, FIG. 36(a) showing the apparatus in a stationary state, FIG. 36(b) showing the apparatus during operation. FIG. 37 is a side view showing the magnetic head positioning mechanism shown in FIGS. 36(a) and (b). FIG. 38 is a perspective view showing a head gimbal assembly in the magnetic head positioning mechanism shown in FIGS. 36(a) and (b). FIG. 39 is a bottom view showing the same mechanism.
The prior art magnetic head positioning mechanism shown in these Figures is called rotary actuator system, in which a magnetic head is driven accurately. As shown in FIGS. 36(a) and 36(b), the mechanism comprises an arm block (or carriage) 113, which includes a plurality of holder arms 111 disposed one above another in the height direction and a movable coil 112 and is mounted on a bearing shaft 114 for rotation in the direction of arrow A.
As shown in FIG. 37, the arm block 113 includes a plurality of holder arms 111 disposed one above another in the height direction. As shown in FIGS. 38 and 39, to the free end of each holder arm 111 is connected a head gimbal assembly 101 (hereinafter abbreviated as HGA), which includes a float-up type or contact type slider 103 having a magnetic head 102, a gimbal spring 104 supporting the slider 103 and a load beam 105 providing a pushing force to the slider 103. The movable coil 112 is provided on the other end of the arm block 113. The movable coil 112 and an external stationary magnetic circuit 116 constitute voice coil motor (hereinafter abbreviated as VCM). When the movable coil 112 is energized by flowing a predetermined drive current through it, it generates a drive force to cause rotation of each holder arm 111 about the bearing shaft 114 so as to drive each HGA 101 along an arcuate orbit in seak directions (as shown by arrows A), thus positioning each head 102 to a desired track on the medium. By the term "positioning" is meant either a seak operation (or tracking) of moving the head from a given track position to a desired track position, or a follow operation (i.e., following) of following desired tracks with the head.
As shown above, in the prior art magnetic head positioning mechanism, a single VCM simultaneously drives a plurality of heads. Therefore, the accuracy of positioning, particularly the accuracy of track following in the following operation, is insufficient, and it is a trend of the mechanism that it is becoming increasingly difficult to apply it to high TPI apparatuses requiring small track pickes of 1 .mu.m or below as noted above.
Recently, researches and investigations of commonly termed two-stage actuators having a drive mechanism for independently driving each head are being made in addition to and independently of arm block (i.e., carriage) driving by VCM.
The two-stage actuators are roughly classified into three kinds, as shown in FIG. 40, in dependence on independently driven parts, that is:
a head drive system in which a plurality of heads are driven independently (FIGS. 40(a) and 40(b)); a slider drive system in which a plurality of sliders are driven independently (FIGS. 41(a) and 41(b); and an HGA drive system in which a plurality of head gimbal assemblies (HGAS) are driven independently (FIGS. 42, 43(a) and 42(b)).
In the head drive system as shown in FIG. 40, an electrostatic drive type linear actuator 202 having a comb structure is buried in a slider 201 having a magnetic head 203 by utilizing micromachine techniques. In the Figure, designated at 204 is a load beam supporting the slider 201.
The slide drive system as shown in FIGS. 41(a) and 41(b), has a structure which is obtained by combining a silicone microgimbal 301 and a planar electromagnetic drive type piggyback microactuator 302. In the Figures, designated at 303 is a load beam, at 304 a slider, and at 305 an R/W amplifier/driver.
The HGA drive systems shown in FIGS. 42, 43(a) and 43(b) are sub-classified into a high stiffness type (FIG. 42) and a high compliance type (FIGS. 43(a) and 43(b)).
In the high stiffness type as shown in FIG. 42, a laminate piezoelectric element 402 is buried in an end portion of an arm block 401, and an HGA 404 is driven by a pair of parallel leaf springs 403.
This high stiffness type has advantages that a servo system can be readily constructed and that a small thickness, compact and high rigidity design is possible. In the Figure, designated at 405 is a movable coil, and at 406 a recording medium.
In the high compliance type as shown in FIGS. 43(a) and 43(b), a small VCM is constructed by disposing a small coil 503, a yoke 506 and permanent magnet 507 in a portion, in which an HGA 502 is connected. More specifically, the HGA 502 is connected to a holder arm 501, which is mounted on a shaft 504 which is in turn rotatably mounted in a cross spring 505. The HGA 502 is driven for rotation by electromgnetic forces of a VCM. This high compliance type is designed with the cross spring 505 disposed in a microactuator bearing part and bent 90 degrees with respect to a plane of driving, thus reinforcing advancement direction rigidity while sacrificing rotation (or drive) direction rigidity. A great drive stroke is thus obtainable with a relatively low current.
The prior art two-stage actuator techniques as described above, however, have the following problems. In the head drive system as shown in FIG. 40, in which the electrostatic drive linear actuator having the comb structure is buried in the slider, high machining accuracy is required. Therefore, the yield is inferior. In addition, ready displacement is caused by shocks exerted in the movement direction.
In the slide drive system as shown in FIGS. 41(a) or 42(b), it is impossible to set a sufficiently large thickness of coil pattern layer for generating induced magnetic field. Therefore, a sufficient drive force can riot be generated, and the system has not yet been in practical use.
As for the HGA drive systems, in the high stiffness type as shown in FIG. 42, although the system permits ready construction of a servo system and small thickness, compact and high rigidity design, a large current is necessary for driving the HGAs, thus leading to extreme deterioration (or rupture in the extreme case) due to changes in ambient conditions such as temperature.
In the high compliance type as shown in FIG. 43, the complicated bearing spring (i.e., cross spring) is undesired from the standpoint of the small size (or small thickness) design of the actuator part, and also poses problems in vibration characteristics, such as pronounced off-track resonance generated by a combination of advancement vibrations and rotational vibrations due to complicated spring structure.